1. Field of the Invention
The present invention relates to a spectrally sensitized silver halide photographic emulsion, particularly, it relates to a spectrally sensitized silver halide photographic emulsion which is useful in methods of obtaining direct positive images by the surface development treatment of an internal latent image silver halide photographic emulsion in the presence of a fogging agent.
2. Description of the Prior Art
Methods of obtaining a direct positive image by the surface development treatment of an internal latent image silver halide photographic emulsion in the presence of a fogging agent, and photographic emulsions or sensitive materials used in such methods, are known and are described in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,675,318, 3,227,552, British Pat. No. 1,151,363, Japanese Patent Publication No. 29,405/68, U.S. Pat. No. 2,592,250, British Pat. No. 1,011,062, and so on.
An internal latent image silver halide photographic emulsion can be defined as one which contains sensitive centers mostly in the interior of the silver halide grains and which forms a latent image mostly in the interior of the silver halide grains resulting from such internal sensitive centers. A photographic emulsion comprising such silver halide grains cannot substantially be developed with a surface developer, that is, a developer which develops only a latent image on the surface of the silver halide grains.
In the above methods of obtaining a direct positive image, a fogging agent can be added to the developer, but better reversal characteristics are obtained if the fogging agent is adsorbed on the surface of the silver halide grains by incorporating it in a photographic emulsion layer or another layer of the sensitive material.
In a silver halide photographic material, a silver halide photographic emulsion is, in most cases, spectrally sensitized. Particularly, in color photographic materials a green sensitive layer and a red sensitive layer as well as a blue sensitive layer are essential, and spectral sensitization is required in order to obtain a green sensitive and a red sensitive layer. In photograhic materials for obtaining a direct positive images, competitive adsorption between a fogging agent and a spectral sensitizing dye occurs and spectral sensitization is inhibited when a fogging agent is incorporated into the photograhic material. One method of solving such difficulty is described in U.S. Pat. No. 3,718,470, i.e., using a spectral sensitizing dye containing in its molecule a substituent having fogging activity. However, the method of imparting fogging activity and spectral sensitivity to one molecule has the defects that the use of an amount thereof sufficient for spectral sensitization is unsuitable for nucleating (fogging) or the use of an amount thereof sufficient for nucleating is unsuitable for the spectral sensitization. We have found a more improved method of obtaining the color sensitization reversal characteristics than the above known method.